This invention, in its preferred form, relates generally to a nuclear fuel assembly grid having interleaved inner grid straps and an outer grid strap forming the perimeter thereof. More particularly, the invention relates to the outer grid strap segments which have improved means for connecting same.
Nuclear fuel assembly grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Grids are made of materials with low neutron absorption cross sections such as stainless steel, Inconel, and alloys of zirconium to minimize grid deformation and loss of structural integrity during irradiation. Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved inner grid straps of egg-crate configuration designed to form cells which accept fuel rods. Vertically cut out slots in the inner straps at the locations of intersection between first inner straps, wherein the slots may be cut from the top of the strap to the middle of the strap, and second inner straps, which are perpendicularly interposed to the first inner straps, wherein the slot may be cut from the bottom of the second strap to the middle of the strap, such that the slots of the first and second inner straps interlock, as is commonly known in the art. The two opposing ends of each of the inner grid straps further are interlocked with the outer grid straps, thereby forming the periphery cells of the grid. Each cell provides support to one fuel rod at a given axial location through the use of relatively resilient springs of various forms. The outer grid strap segments are attached together and peripherally enclose the inner grid straps to impart strength and rigidity to the grid. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length.
In pressurized water reactors (PWR), typically each grid is held in place along the fuel assembly by attachment to control rod guide thimbles. In a boiling water reactor (BWR), typically a can surrounds the fuel rods and grids. In the BWR, each grid is held in place along the fuel assembly length by a special locking rod. In one conventional BWR arrangement, at a given grid vertical location in the fuel assembly, there are four horizontally aligned grids in a square array separated by a water cross and surrounded by the can.
Typically, in an outer grid strap, there are four equivalent outer grid strap segments, each defining a planar end of the grid. Each outer grid strap segment is joined to another outer grid strap segment, forming generally a right angle, at the respective ends thereof. The outer grid strap segments are generally congruent with each other, each generally having coinciding inclined or otherwise curved ends. The respective ends of the outer grid strap segments overlap, and are held in place by various fixtures during manufacture of the grid, thereby controlling tolerances of the grid. The overlapped ends are fixedly bonded by either spot welding or by a cosmetic fusion pass or seam weld of the outside ridge of the overlapping ends if the outer grid strap is made of zircaloy, stainless steel or other material that may be welded, or by brazing the inside and outside ridges of the overlapping ends if the outer grid straps are made of Inconel or nickel plated Inconel or other nonweldable material. Representative welding apparatus and methods and fixtures, for holding grids during the welding process are disclosed in U.S. Pat. Nos. 4,556,776, 4,541,055 and 4,539,738, all assigned to the same assignee as the present invention.
Designers are constantly seeking to improve the means of manufacture of the grids. Areas of interest include reducing the number of welds required to form the grid, and meeting the stringent design envelope, or tolerances, on dimensional parameters of the grid. Further considerations include retaining the structural rigidity of the grid, and reducing the amount of material in the grid for minimizing neutron absorption and coolant flow interference by the grid. Therefore, what is needed is an outer grid strap with improved means for connecting the segments of the strap, taking into account the foregoing considerations.